It has been known for a very long time to produce covered elastic yarns, particularly by the so-called "single-covering" technique. For this purpose, the elastane yarn is first stretched by three to five times, and then a so-called "covering" yarn, especially a textured yarn, is wound round this in preferably contiguous coils. In practice, the covering twist, which varies as an inverse function of the count of the covering yarn, is of the order of one thousand to three thousand revolutions per meter, since it is desirable to obtain substantially contiguous coils. Thus, when the finest possible single-covered yarns with a minimum of swelling are to be produced, for instance the yarns required for the production of stocking legs, it is customary to use elastane yarns which are relatively fine, that is to say of the order of ten to forty decitex hereinafter dtex, and which are covered with a synthetic yarn, raw or textured by false twist, of ten to thirty dtex, covered at twists of the order of one thousand five hundred revolutions per meter. Although this technique is in very widespread use, it nevertheless has some appreciable disadvantages, such as the high cost price and the impossibility or difficulty of obtaining windings without a knot of considerable weight.
The Document FR-B-2,561,676 of the Applicants provided a technique which involves covering the tensioned elastane yarn with a partially stretched multi-filament synthetic yarn at a covering twist of one third of the usual twist, then, in a separate phase, completing this twist by means of a double-twist spindle, and finally, in a likewise separate third phase, subjecting the covered yarn to additional residual hot stretching. This technique gives covered yarns suitable for the production of stockings and tights for which the finest possible yarns are sought. However, to obtain a correct covering of the elastane yarn and thereby protect it against unravelling, the yarn has to be twisted at twists of the order of two thousand revolutions per meter. The result is that this technique, although well developed, still remains costly.
It has also been proposed to knit or weave bare tensioned elastane yarns. This technique, although available for a long time, has not undergone further development because of the excessively high production cost resulting particularly from the high percentage of finished products such as panties of second quality.